One working on student is an animal (4)
I believe the answer is:
lion
'an animal' is the definition.
(thesaurus)
'one working on student' is the wordplay.
'one' becomes 'i' (Roman numeral).
'working' becomes 'on'.
'on' says to put letters next to each other.
'student' becomes 'l' (as in L-plates for learner drivers**).
'i'+'on'='ion'
'ion' put after 'l' is 'LION'.
'is' acts as a link.
(Other definitions for lion that I've seen before include "Wild cat" , "Carnivorous 6" , "cowardly character in Oz" , "King of the jungle" , "The -, the Witch and the Wardrobe" .)